The present invention relates to exhaust gas transport devices and systems as used with internal combustion engines and, more particularly, to improved butterfly valves and related butterfly valve assemblies as used in a vehicle exhaust gas recirculation system.
EGR systems are designed to recirculate exhaust gas generated by an internal combustion engine back into an engine intake stream. Since the exhaust gas exiting the engine is already combusted, it does not combust or burn again when it is recirculated into the combustion chamber, thereby acting to displace some of the normal intake charge. The effect of adding such exhaust gas to the intake charge operates to chemically slow and cool the combustion process by several hundred degrees, thereby acting to reduce NOx formation.
For this reason, EGR systems have gained widespread acceptance and application for use with many different types of internal combustion engines. In order to use an EGR system, a valve must be used to control the flow of exhaust to the intake charge. It is desirable to use a butterfly valve in such an EGR system, and the use of butterfly valves in EGR systems are known. However, EGR systems comprising traditional butterfly valves are not without problems that must be overcome.
For example, typical butterfly valves are disposed within a circular valve housing having a constant diameter, i.e., having a straight-bore geometry. The use of a butterfly valve with such a straight-bore geometry housing provides a flow characteristic (from a device near constant upstream pressure and a variable downstream pressure such as a diesel engine) that is non-linear relative to the positioning of the valve, i.e., with a decreasing positive sloped pressure curve. In this traditional butterfly valve assembly a greater percentage of flow across the valve is achieved within the first 30 percent of opening of the valve than 30 percent flow. In fact, as the valve is opened the remaining amount of the way from the 30 percent position (i.e., from 30 to 95 percent), there is not as much increase in the flow (i.e., there is not a corresponding 65 percent increase in flow).
Thus, the use of such traditional butterfly valve assemblies in EGR service do not provide a high degree of flow control sensitivity at the just-opened valve position. For use in EGR system service, however, there is a need for a butterfly valve assembly that is capable of providing an improved degree of flow control sensitivity near the closed valve position.
Additionally, the use of traditional butterfly valves and valve assemblies in EGR systems oftentimes do not provide a high degree of sealing at the level of precision that is required for an EGR Valve application. For example, a typical butterfly valve in such service does not have a full 90 degrees of motion, and the sealing surface of such valve is positioned between an outer edge of the valve flapper and the wall of a bore (or valve housing) within which the valve is situated. The undesired leakage of such traditional butterfly valves placed into EGR service is a key factor in producing particulate emissions associated with a diesel engine. There is, therefore, a need for a butterfly valve and assembly that is designed in such manner so as to provide an improved internal seal (i.e., a seal between the valve and a valve seat) when placed into a closed position, thereby reducing leakage thereby to help control/reduce particulate emissions.
Additionally, it is known that traditional butterfly valves placed into EGR system service tend to produce an unwanted heavy soot buildup on adjacent hardware, such as an engine block, water pump, oil cooler, turbocharger, etc. This heavy soot buildup is caused by the external leakage of high pressure (pre-turbine) exhaust gas from the valve to outside atmosphere. Such external leakage of exhaust gas from the butterfly valve often occurs through the valve assembly controlling the valve inside of the bore. There is, therefore, a need for a butterfly valve and valve assembly that is designed to reduces the potential for external leakage of high pressure exhaust gas therefrom and to the atmosphere.
Butterfly valves and related valve assemblies/housings, constructed according to the practice of this invention, to address that above-identified needs generally comprise a flapper element that is rotatably disposed within an annular bore of a valve housing. The valve may be configured to provide an improved degree of gas flow control sensitivity from a closed to a partially opened position through the use of projections that extend radially inwardly from the housing bore, are positioned at diametrically opposed positions within the bore, and that are specifically configured to provide a gradual increase in gas flow across the valve as the flapper is moved from a closed position to a partially opened position.
In another embodiment, the butterfly valve is configured to provide an improved internal seal when placed in a closed position to reduce or eliminate the unwanted passage of exhaust gas therethrough. In such embodiment, the butterfly valve is configured with the valve housing having projections that extend radially inwardly a distance therein, that are positioned at diametrically opposed positions in the bore, and that include faces that are sized and shaped to cooperate with respective opposed surfaces of the flapper to seal off gas flow within the bore when the flapper is placed into a closed position.
In still another embodiment, the butterfly valve is configured to provide an improved seal to control leakage of exhaust gas from the valve to the external environment. In such embodiment, the butterfly valve is configured comprising a flapper that is positioned on a shaft extending through the housing. A bushing surrounds the shaft, is interposed between the shaft and the housing, and extends outwardly from the housing. The shaft is coupled to an arm outside of the housing. The bushing and arm are configured having complementary surfaces to provide a seal therebetween that functions to minimize or eliminate the unwanted leakage of gas from the valve.